The crystal structure of apophyllite, a mineral with the ideal formula KCa4(Si4O10)2F ·8H2O, has been refined using three-dimensional neutron diffraction data. 671 independent reflections were observed on a 4-circle diffractometer. Of these 516 had observable intensities. A least squares refinement, with anisotropic temperature factors, led to an R index of 0.046. However, a difference Fourier synthesis revealed a region of negative scattering density in the vicinity of the fluorine atom. Accordingly a further refinement was carried out assuming a model in which 1/8 of the water molecules are replaced by OH—ions, with the remaining proton bonded to fluorine to form an HF molecule. This model refined quickly to an R value of 0.037. The water molecules and OH ions are hydrogen bonded to the silicate framework. A preliminary mass spectrometric analysis of the gases evolved when apophyllite is heated revealed that the higher temperature specific-heat anomaly is due to the evolution of hydrogen fluoride. This suggests that the structural formula is KCa4(Si4O10)2 F1–x(HF)x·[(H2O)8–x(OH)x] with x≈1.